Just a quick question regarding the sources used for investigating entanglement (of photons). There are two types of production method as I understand it: ('Fundamentals of quantum optics and quantum information', Peter Lambropoulos, David Petrosyan, 2006, Springer): Page 242 "In fact, most of the experimental tests of Bell's inequalities have been performed using polarization-entangled photon pairs. These include a series of pioneering experiments by Aspect and coworkers, using atomic radiative cascade, as well as a number of experiments by several teams using nonlinear crystals to realize spontaneous parametric down-conversion. In the former experiments, a high-efficiency source of pairs of photons ... was obtained... The polarization entanglement of the photons comes about because of angular momentum conservation. Since relative to photons, the atoms are massive objects, their recoil during photon emission is negligible. Therefore... the propogation directions of the two photons are also strongly correlated, due to energy and momentum conservation. In the experiments with nonlinear crystals... here again, angular momentum is conservation imposes polarization entanglement between the photons, while phase-matching conditions result in a finite angle between propogation directions of the photons, which make it possible to redirect each photon to its own measuring apparatus" So my questions is: is there anything (other than ease-of-use for the particular experiments conducted) that prevents us from swapping source? I.e. could Aspect have used SPDC and the Quantum Erasor teams used a radiative cascade source and achieved the same results as they did? This is probably sounds like a really dumb question but I just feel like I'm missing something fundamental here but I can't see what?